1. Field of the Invention
This invention relates to an electrochemical process for oxidizing thiocyanate (SCN.sup.-). In particular, this invention relates to a process for recovering cyanide (CN.sup.-) from aqueous solutions containing thiocyanate by controlled partial electrooxidation of thiocyanate. This invention further relates to a process for electrochemically oxidizing thiocyanate in an aqueous solution to relatively harmless reaction products.
2. Description of the Prior Art
Aqueous solutions containing thiocyanate arise from many industrial processes, the principal sources being hydrometallurgical processing of gold and silver ores and concentrates and certain unit operations related to base metal processing. Very large volumes of effluent containing somewhat lower levels of thiocyanate eminate from coking operations either from the quenching waters or gas cleaning installations. The refining of petroleum produces dilute thiocyanate solutions and thiocyanate is a common component of many inorganic waste streams generated by the chemical industry. Waste effluents containing thiocyanate are environmentally objectionable because in the natural environment thiocyanate is oxidized by various pathways yielding highly toxic cyanide compounds.
It is helpful to consider an example of a typical thiocyanate containing waste liquor that could be treated by the present process. In gold recovery by cyanidation of sulfidic concentrates obtained by froth flotation of copper ore tailings, the waste liquor effluent may contain 1000-1200 milligrams of CN.sup.- per liter and 1200-1400 milligrams per liter of SCN.sup.-. The presence of thiocyanate in the effluent represents a significant loss of reagent cyanide.
The formation of thiocyanate is a result of the release of sulfide (S.sup.2-) present in compounds of copper, iron, nickel and other metals during the cyanidation leaching of the tailings. Sulfide undergoes chemical oxidation in the oxygen rich leach liquor to form a series of oxysulfur species including thiosulfates and thionates. It is believed that thiocyanate is formed by reaction of cyanide with thionates. A reaction suggested for the formation of thiocyanate by the action of trithionate ion (S.sub.3 O.sub.6.sup.2-) with cyanide is shown in equation (1). EQU S.sub.3 O.sub.6.sup.2- +CN.sup.- .fwdarw.SCN.sup.- +S.sub.2 O.sub.6.sup.2-( 1)
In addition to the irreversible consumption of reagent cyanide, there is evidence to suggest that the presence of thiocyanate in gold cyanidation solution inhibits the oxidation of gold and therefore retards its solubilization. This effect could possibly be due to the formation of unstable gold sulfides on the metallic gold surface thereby reducing the rate of mass transport of the reactants, cyanide and dissolved oxygen resulting in a reduction of the gold leaching rate. A common practice in gold mills which serves to maintain the thiocyanate at an appropriate and acceptably low level is to discharge up to 20% of the thiocyanate fouled leach liquor from the cyanidation circuit per day. The remaining liquor is then regenerated by addition of reagent cyanide. Let us assume for the purposes of this example that the volume of fouled leach solution discharged per day is 250 metric tons. This represents approximately 350 kilograms of free and complexed cyanide per day.
Another source of waste effluent occurs in the processing of a concentrate fraction obtained from complex zinc-copper-lead sulfide ores. In this example, it is necessary to use a cyanide concentration of twenty times the conventional level in order to effect dissolution of contained silver values. Under these conditions, it is found that a significant fraction of the cyanide is converted to thiocyanate. The barren discharge solution can be acidified allowing the expurgation of cyanide as hydrocyanic acid (HCN). The cyanide depleted residual acidic solution may contain up to 1000 milligrams/liter of thiocyanate. The silver recovery process may produce up to 1800 kilograms of thiocyanate per day.
The two examples given above demonstrate the large quantity of thiocyanate bearing waste liquor produced by cyanidation of sulfide ores and concentrates. The conventional method of processing this type of effluent (aside from natural oxidation in holding ponds which is reported to be relatively slow when compared to the natural oxidation of cyanide) is by chemical oxidation using aqueous hypochlorite or using chlorine gas and aqueous caustic--the latter is usually termed alkaline chlorination. The stoichiometry for the alkaline chlorination of thiocyanate to cyanate (CNO.sup.-) and sulfate (SO.sub.4.sup.2-) is often represented by equation (2). EQU SCN.sup.- +4Cl.sub.2 +10OH.sup.- .fwdarw.CNO.sup.- +8Cl.sup.- +SO.sub.4.sup.2- +5H.sub.2 O (2)
The cyanate species (CNO.sup.31) may undergo further oxidation with additional chlorine and base but will also dissociate via a hydrolysis reaction producing in receiving waters, ammonia and carbonate. Using the stoichiometry of equation (2), an estimate of the chemical requirements can be made for treating by conventional means the thiocyanate contained in the effluent of example 1. If a typical 10% reagent excess is assumed, approximately 0.85-1.0 metric tons per day of chlorine is required together with 2.3-2.7 metric tons of sodium hydroxide per day (a portion of the base requirement may already be available in the effluent). The treated waste would contain approximately 2.4-2.8 metric tons per day of sodium chloride which often is unacceptable in receiving waters. For the purposes of comparison, the chemical requirements for oxidation of 300 kilograms per day of cyanide would be 0.9 metric tons per day of chlorine and 1.0 metric ton per day of sodium hydroxide. The stoichiometry of the alkaline chlorination of cyanide is given by equation (3). EQU CN.sup.- +Cl.sub.2 +2OH.sup.- .fwdarw.CNO.sup.- +2Cl.sup.- +H.sub.2 O (3)
These estimates of reagent requirements indicate that the oxidation of thiocyanate by chemical means is an inherently expensive and hazardous proposition and is generally regarded as being much more expensive than alkaline chlorination of the cyanide which often accompanies the thiocyanate oxidation.
It is an object of the present invention to provide a process whereby thiocyanate can be electrochemically oxidized more economically than by conventional means and to recover, for credit and reuse, cyanide which forms as an intermediate product of the electrooxidation. The process of the present invention can be carried out on a batch or continuous basis with a variety of effluent compositions. With many thiocyanate effluents no chemical pretreatment such as pH adjustments or adjustment of the buffer index or capacity of the effluent before electrochemical treatment is required. Also, when thiocyanate or cyanide is treated in the conventional manner by chemical oxidation, the waste contains a large amount of sodium chloride and may very well contain undesirable levels of free chlorine or sodium hydroxide from chemical overdosage. In addition, when treated in the conventional manner, the volume of the effluent may be considerably increased by the large volume of reagents added.